1. Field of the Invention
The present invention relates to an electro-deposition coated member capable of improving electromagnetic wave shielding effect, that can be used as a housing for electronic machinery including optical instruments such as cameras, sound instruments such as CD players, and office automation machinery, which are sources from which electromagnetic waves are generated. It also relates to a process for producing such an electro-deposition coated member, and an electro-deposition coating composition used therefor.
2. Related Background Art
In recent years, as electronic circuits have been made more small-sized, complicated and precise, the misoperations and noise caused by electromagnetic waves generated from other component parts and circuits have presented major problems. The electronic circuits themselves also generate electromagnetic waves, and also offer an important problem on their influences on surroundings. In order to prevent these problems, it is sought to shield electronic circuits from invasion or radiation of electromagnetic waves.
As methods for shielding electromagnetic waves, a method is conventionally known in which a circuit substrate is surrounded with a metallic housing comprising a conductive material. However, as the products are recently made small-sized and light-weight, it has been prevailing to use a housing comprised of a plastic material. As a method of making such a plastic housing conductive, it has been prevailing to use spray coating using a conductive coating composition. Other methods are also used which include zinc spray coating, electroless plating, vacuum deposition and conductive plastic coating.
The conventional methods, however, have the following disadvantages.
The conductive coating composition for spraying can achieve no sufficient electromagnetic wave shielding effect unless a conductive filler is contained in an amount of not less than 60 parts by weight and moreover a coating thickness is not less than 30 .mu.m in the case of a copper filler and not less than 50 .mu.m in the case of a nickel filler. For this reason, this coating composition is not suitable for decorative coating that provides an exterior coat on a housing.
In instances in which metal powder is used as the fillers, the metal powder has so large a specific gravity that it is required for the powder to be again dispersed when coating compositions are used, which, however, is not easy. To solve this problem, Japanese Patent Application Laid-open No. 59-223763 discloses a conductive coating composition for electromagnetic wave shielding in which Ni-coated mica powder is used as a conductive filler. This coating composition also can not achieve a sufficient electromagnetic wave shielding effect unless a coating is formed in a large thickness of 50 .mu.m or more.
In addition, in housing with complicated shapes, the coating thickness tends to be non-uniform, often resulting in an insufficient shielding effect.
As for the zinc spray coating, it must give a coating thickness of as large as from 50 to 100 .mu.m in order to ensure the shielding effect, and also has a difficulty in adhesion to substrates. For this reason, it becomes necessary to provide steps for blast finishing, etc. In addition, there is still a problem in mass productivity because of a work environment worsened by zinc vapor gas.
In regard to the electroless plating, an electromagnetic wave shilding effect can be obtained when, for example, a copper coating is formed in a thickness of 1.0 .mu.m to 1.5 .mu.m or more. Since, however, the whole housing is plated, it becomes indispensable when used as a housing of a product, to form a coating film on the plated surface to improve the nice-looking appearance so that the commercial value can be enhanced. In doing so, however, there is a problem of the poor adhesion between the film surface formed by plating and the coating surface formed by coating. In particular, mere plating with copper may cause changes with time to bring about corrosion, resulting in a lowering of performances. Hence, the copper-plated surface must be subjected to nickel plating so that the quality can be prevented from being lowered. Moreover, since this nickel plating may greatly impair the adhesion to the coating film, the coating must be carried out using very limited materials such as special coating compositions as exemplified by Origiplate Z (available from Origin Electric Co., Ltd.). This greatly effect cost and can not be mass-productive.
On the other hand, a conductive plastic housing is known, which is formed of a mixture of a resin and a conductive filler such as metal powder with particle diameters of several tens or more .mu.m or metal fiber. The resulting plastic housing, however, has too seriously uneven a surface to be usable as an exterior member if it is used in the state of a molded product untreated or unfinished. Thus, there is the problem that decorative coating must be applied in order to attain commercial value. In addition, because of poor conductivity, any secondary finishing becomes necessary for achieving perfect electromagnetic wave shielding, which can not be mass-productive. Moreover, since conductive plastic materials themselves are expensive, there is also a limit on its practical utilization.